A secondary battery is contributing to reduction of waste compared to a primary battery because it is possible to use a secondary battery while repeating charge and discharge many times. Also, a secondary battery is widely used as a power source for a portable device which cannot be connected to an AC power source and a backup power source when an AC power source is disconnected or stopped. In recent years, the expansion of application range of a secondary battery has been studied as exemplifying a vehicle application, a backup power source application such as a solar cell, and a power leveling application in a solar cell, etc. This expansion increases the demand for the improvement of performances such as capacity, temperature characteristics and safety in a secondary battery further and further.
Among secondary batteries, a nonaqueous electrolyte solution secondary battery is a secondary battery which performs charge and discharge through the movement of lithium ion between positive and negative electrodes. Because an organic solvent is used as an electrolyte solution, it is possible to obtain the larger voltage than a Ni—Cd secondary battery or a nickel hydrogen secondary battery using an aqueous solution as an electrolyte solution. Currently, in a commercially available nonaqueous electrolyte solution secondary battery, a lithium-containing cobalt composite oxide or lithium-containing nickel composite oxide is used as a positive electrode active material, a carbon-based material or lithium titanate, etc. is used as a negative electrode active material, and a solution prepared by dissolving a lithium salt such as LiPF6 or LiBF4 in an organic solvent such as a cyclic carbonate or a linear carbonate is used as an electrolyte solution.
The average operating potential of a positive electrode active material is approximately within a range of 3.4 to 3.8 V with respect to a lithium metal potential, and the maximum peak potential thereof during charge is approximately within a range of 4.1 to 4.3 V. On the other hand, the average operating potential of a carbon-based material which is a negative electrode active material is approximately within a range of 0.05 to 0.5 V with respect to a lithium metal potential, and the average operating potential of lithium titanate is approximately 1.55 V. By combining these positive and negative electrode materials, the battery voltage falls within a range of 2.2 to 3.8 V, and the maximum charge voltage falls within a range of 2.7 to 4.3 V.
One of the proposed methods of further improving a capacity is a method of using LiMn1.5Ni0.5O4 in which the maximum peak potential during charge is within a range of 4.4 to 5.0 V. However, when using a carbonate-based solvent which has been conventionally used, a carbonate-based solvent causes an oxidation reaction at a positive electrode during charge, which can results in the deterioration of battery performance and gas generation.
By contrast, another proposed method is a method of using sultone or a sulfone-based compound as a solvent. However, there were several problems that sultone or a sulfone-based compound has a high viscosity, a low solubility to a lithium salt, and a high reactivity to a negative electrode as compared with a conventional solvent. In addition, when using sultone or a sulfone-based compound as a solvent, the initial charge and discharge efficiency and the gas generation amount are poorer than those of the currently used nonaqueous electrolyte solution secondary batteries, and further improvement is required.
Sultone and a sulfone-based solvent have a high viscosity and a high dielectric constant. When an electrolyte solution is prepared by dissolving a lithium salt, the ion conductivity decreases compared with an electrolyte solution prepared by using a conventional carbonate-based solution. Also, sultone and a sulfone-based solvent have a poor impregnation property to an electrode as compared with a conventional carbonate-based solvent. For this reason, there was also the problem that the nonaqueous electrolyte solution battery produced by using sultone or a sulfone-based solvent has poor output characteristics and cycle performance as compared with a nonaqueous electrolyte solution battery produced by using a conventional carbonate-based solvent. By contrast, the electrolyte solution prepared by mixing sultone or a sulfone-based solvent with a carbonate-based solvent has been proposed. However, a carbonate-based solvent tends to be preferentially oxidized, and unignorable gas generation can occur at an enough mixing amount to obtain the sufficient decrease in viscosity.